Digital data may be stored on flexible or "floppy" disks or on hard or Winchester-type disks by the magnetization of successive small areas on the magnetic surface of the disk, by a magnetic head or "slider", as the disk rotates. The density of digital storage on hard disk memory systems is in the order of 10 or 20 times the density achieved with floppy disk memory systems.
In the field of hard or Winchester-type disk systems, the disks are normally formed of aluminum, and have a magnetizable coating on their upper and lower surfaces.
Rigid or Winchester-type magnetic storage disks were originally relatively large in diameter, but in the last few years the size has been reduced, so that 51/4 inch disk systems are now widely available. Further, industry standard dimensions have been established for 51/4 inch Winchester-type disk units, and these dimensions include a height of 3.25 inches or 82.6 millimeters, a width of 5.75 inches, or 146 millimeters, and a depth of 8.00 inches, or 203 millimeters. In addition, the industry standard for weight is six pounds, or 2.7 kilograms. Environmental standards have also been established, with the operating temperature extending from 10 degrees C. to 50 degrees C. and the non-operating or storage temperature extending from -40 degrees C. to +65 degrees C.
By using a flat type of drive motor, it has previously been possible to mount five hard disks of the 51/4 inch size within the industry standard dimensions mentioned hereinabove. This type of configuration would provide a storage capacity in the order of 170 megabytes.
It appears, however, that computers are "insatiable" in their demands for additional rapid access storage, and accordingly, there is a demand for 51/4 inch disk drives with larger capacities. One way of increasing the number of disks which may be included within the industry standard dimensions is to locate the drive motor centrally with respect to the storage disks, with the disks being directly secured to the motor rotor. However, the motor involves magnetic material, usually steel, and the storage disks are normally made of aluminum, coated of course with a magnetizable material. Unfortunately, the thermal coefficient of expansion of iron or steel is in the order of six to nine microinches per inch per degree Fahrenheit; whereas the thermal coefficient of expansion of aluminum is in the order of 11 to 13 microinches per inch per degree Fahrenheit. Over the range of temperatures through which the disk drive is to be operative, if the aluminum disks were to be rigidly mounted to an outer steel cylinder forming part of the motor rotor, the difference in thermal expansion would cause the disks to flex or deform, so that the precision digital memory would not operate reliably. In this regard, it is noted that one of the disk surfaces is normally reserved for servocontrol, so that normal and uniform expansion and contraction of the aluminum disks, without deformation or buckling, causes no problem. However, if the disks flex, or depart from their normal flat surface configuration, reliability will suffer severely, or the units may become wholly inoperative for digital storage. It is also noted that, if steel bearings were to be used on a steel shaft, to directly mount an aluminum disk assembly, the differential thermal expansion of the aluminum and steel could drastically change the bearing pre-load and significantly alter the dynamic characteristics of the disk drive.
Accordingly, a principal object of the present invention is to overcome the problems arising from differential thermal expansion, for example, when aluminum memory disks are employed with a central motor having a steel frame or outer cylindrical portion of the rotor, as outlined hereinabove.